This invention relates to a liquid electrolyte fuel cell such as a molten carbonate fuel cell and, in particular, to an end cell assembly for use at the positive and negative ends of a stack of such fuel cells.
As shown in FIG. 1, a fuel cell generally comprises an anode and cathode separated by an electrolyte. An anode current collector is provided adjacent to the anode, opposite the electrolyte, through which fuel is allowed to pass. Similarly, a cathode current collector allowing passage of oxygen is provided adjacent to the cathode and opposite the electrolyte.
Fuel cells of this type using carbonate as the electrolyte are well-known and have been described in numerous publications and patents. In a molten carbonate fuel cell, the carbonate electrolyte comprises an alkali metal carbonate material, such as lithium or potassium carbonate, in a particulate matrix of inert ceramic material, such as lithium aluminate. At the operating temperature of the molten carbonate fuel cell, which is approximately 650° C. (1200° F.), the carbonate electrolyte is an ionically conductive molten liquid.
With fuel introduced at the anode electrode and oxidant introduced at the cathode electrode, the fuel is oxidized in an electrochemical reaction at the interface between the electrodes and the electrolyte. This releases a flow of electrons between the anode and cathode, thereby converting chemical energy into electrical energy. The anode and cathode electrodes are each preferably made of a porous metal such as porous nickel powder or nickel oxide that is sufficiently active at cell operating temperatures to serve as catalysts for the anode and cathode reactions, respectively.
A single fuel cell as shown in FIG. 1 produces relatively low voltage. In order to generate higher voltages, individual cells are arranged in series as a fuel cell stack. As shown in FIG. 1, a separator plate, preferably made of stainless steel, is provided to separate each fuel cell from adjacent cells in the stack. As used herein, the term “end cell” is defined as either of the fuel cells at the positive or cathode and negative or anode ends of the stack, each of which provides structural termination.
A significant problem associated with the fuel cell stack configuration is loss of electrolyte preferentially from the cells closest to the positive end of the stack and gain of electrolyte mostly by the cells closest to the negative end of the stack. Mainly, two processes cause the electrolyte loss from the positive end in excess of the central cells. The first process is liquid electrolyte creepage onto the structurally terminating end plate, which is adjacent to the end cell. The second process, migration of electrolyte, causes electrolyte to flow in films along the surfaces of the stack toward the negative or anode end. As a result, fuel cells at the positive end of the stack lose more electrolyte compared to the central cells and cells at the negative end gain electrolyte. The effects of electrolyte migration are the most severe in the end cells, which are positioned closest to the positive and negative ends of the stack. Depletion of electrolyte from the positive end cell by creepage and migration leaves gas pockets in the electrolyte matrix. This results in an irreversible increase in internal electrical resistance of the end cells, significant voltage drop, and deterioration of long-term end cell performance. Also, electrolyte migration towards the negative end may cause flooding of the negative end cell and loss of performance and long-term stability.
Accordingly, it has been recognized that inhibiting electrolyte migration and the associated increase in electrical resistance of the end cells can enhance the durability of a carbonate fuel cell. Various structures have been proposed to slow or inhibit electrolyte migration. Specifically, efforts have been made in this regard to provide a “half-cell” anode, which refers to an anode section of a fuel cell including an anode reaction gas flow path, an anode electrode and a gas sealing structure adjacent to the end cathode, and a “half-cell” cathode (with a structure similar to the “half-cell” anode) adjacent to the end anode in a fuel cell stack (U.S. Pat. No. 5,019,464 to Mitsuda et al.). The “half-cell” anode in this design stores additional electrolyte that replenishes the electrolyte lost from the cell closest to the positive end of the stack by migration and other mechanisms. The “half-cell” cathode placed adjacent to the negative end regular cell anode absorbs the extra electrolyte that moves to the negative end of the stack. This arrangement prevents flooding of the cell closest to the negative end of the stack and delays depletion of electrolyte from the end cell at the cathode end, but electrolyte migration is not eliminated and the “half-cell” anode must be periodically replenished with electrolyte. Furthermore, the “half-cell” anode placed at the positive end of the stack has limited electrolyte storage capacity. The anode is made of Ni which is non-wetting to the electrolyte, or not conducive to the electrolyte flowing across it, and the anode porosity is also low, i.e., less than 55%, because of structural strength considerations.
Other efforts have been made to substantially block creepage of molten carbonate electrolyte, such as by providing an electrolyte creepage barrier comprising a thin layer of material which is poorly wet by the liquid electrolyte, for example, gold foil, sandwiched between ceramic layers at the negative or anode end of the stack (U.S. Pat. No. 4,704,340 to Kunz); and forming either of the anode or cathode electrodes with ceramic particles having pore size such that capillary forces urge electrolyte into discrete portions of the electrode for storage (U.S. Pat. No. 4,548,877 to Iacovangelo et al.). U.S. Pat. No. 5,110,692 teaches a technique to retard electrolyte movement from the positive end to the negative end by incorporating ceramic electrolyte flow barriers within the gasket body.
Additional efforts have been made in this regard to mitigate against electrolyte migration and consumption by replenishing consumed electrolyte and maintaining the electrolyte at a consistent composition. Particularly, U.S. Pat. No. 4,980,248 to Fujita teaches a molten carbonate fuel cell in which a lithium-containing composite oxide reacts with oxidizing gas flow such that the electrolyte which has been consumed is replenished and maintained at a consistent composition. In addition, U.S. Pat. No. 4,591,538 to Kunz teaches a binary electrolyte with uniform lithium to potassium molar ratio during operation along the length of the fuel cell stack.
A second problem associated with end cells in a fuel cell stack is the increase in end cell electrical resistance due to shrinkage or deformity of cell components at stack operating temperatures. Common molten carbonate stack designs include rigid, thick end plates to which is directly applied an appropriate compressive loading force for adequate sealing and good electrical and thermal conductivity between adjacent cells and components within the stack. At normal stack operating temperatures, and particularly during startup and shutdown of a fuel cell stack, temperature gradients form between the opposite surfaces of the end plates and may cause the end plates to deform. Additional mechanical mismatch may occur during operation of the stack, particularly in the cathode or positive side end cell, due to cathode shrinkage. Carbonate fuel cell cathode shrinkage is known to occur slowly with operation.
The cathode “half-cell” is usually constructed from a corrugated current collector with a cathode attached to it. The corrugated current collector extends over the entire cell area. However, the cathode extends over less than the entire area, since it does not extend to the wet seal edges. Therefore, the wet seal thickness must be matched to the cathode thickness to maintain the flatness of the cell structure. In the prior art, a flat shim made of sheet metal is inserted under the flap of the wet seal to account for the difference in thickness of the active area and the wet seal. As the cathode shrinks, the cell compressive pressure shifts from the active part of the cell to the wet seal area. This lowering of compressive force in the active area causes loss of electrical contact at various locations within the end cell and results in non-uniform application of the compressive force across the fuel cell. Once electrical contact loss occurs, recovery of the original electrical conductivity at the interface is unlikely, even if the original distribution of compressive forces returns.
Some efforts have been made to reduce electrical contact loss in the end cell. Specifically, it has been proposed to use an electrically conductive flexible thin end plate membrane in combination with semi-rigid insulation layers at the positive and negative ends of the fuel cell stack in order to maintain a substantially uniform compressive loading pressure across the plane of fuel cells during normal operation despite thermal distortion (U.S. Pat. No. 5,009,968 to Guthrie et al.). Electrical contact is thereby maintained in the end cell between the flexible conductive end membrane and the electrodes. However, electrolyte migration and the associated increase in resistance are not inhibited by the end cell structure of this patent.
It is therefore an object of the present invention to provide an end cell for storing electrolyte in a molten carbonate fuel cell that does not suffer from the above disadvantages.
It is a further object of the present invention to provide an end cell for use at the positive and negative ends of a molten carbonate fuel cell stack, the end cell at the positive end having a ribbed electrode reservoir that maximizes electrolyte storage capacity and the end cell at the negative end having a ribbed electrode sink that maximizes sink capacity.
It is further objective of this invention to provide a soft, compliant and resilient, highly electrically conductive separating surface between the end plate and the electrolyte reservoir at the positive end of the stack, and between the end plate and the electrolyte sink at the negative end of the stack, to alleviate the resistance increase caused by distortion and inadequate contact between the hard end plate and the cell packages in contact with it, eliminating contact loss in the end cell.
It is a further objective of this invention to provide a soft, compliant and resilient wet seal in the cathode side of the inactive end cell to ensure that when the inactive cathode shrinks during operation of the fuel cell, the cell compressive force is distributed to the cell active area and is not disproportionately transferred to the wet seal area.
It is still a further object of the present invention to provide an improved end cell for electrolyte storage that eliminates the above-mentioned problems associated with electrolyte migration and that enhances and extends long-term end cell performance.